The American Heritage® Dictionary of the English Language, Fourth Edition, defines “fluid” as: “A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; a liquid or gas.” The term “fluid-material” in the context of the present invention refers from herein after to a material that supports or at least does not hinder the growth of microorganisms and is in either a liquid state, a gaseous state or in a mixture of liquid state and gaseous states. Examples of fluidic liquids: water and oil. Example of fluidic gasses: free environmental air and water vapor. An example of liquid and gas mixture: well-aerated-water.
“Water” in the context of the present invention refers to potable water, recreation-utilized water such as lake, pool and seawater, to seawater and brackish water used in desalination processes, and to municipal and industrial wastewater. “Water” also referrers to water used for growing sea and fresh water organisms such as algae, fish, clams and crabs in tanks, aquariums and pools.
The term “contaminates” refers from herein after to solid-state particles suspended in a fluid-material as well as to bio-degradable organic and inorganic substances dissolved in a fluid-material. Typically, but in no way limited to, organic substances are proteins, sugars and lipids. Typically, but not limited to, inorganic substances are nitrates and phosphates. In gaseous state materials the previously referred to substances are typically dissolved in water vapor; the vapor being part of the gaseous materials. The suspended solid-state particles are referred to from herein after as: “suspended solid particles” or “SS particles”.
For public health, environmental considerations and esthetic reasons contaminants are commonly separated and removed in various domestic and industrial processes and procedures. In some processes it is desired to remove both categories of contaminants: the SS-particles and the dissolved bio-degradable substances. In other cases it is desired to remove only one of the two listed contaminant categories. An example for the removal of both contaminant categories is the treatment of municipal wastewater for environmental disposal or agricultural reuse. Examples for the removal of (only) SS-particles are the pretreatment of seawater prior to desalination by reverse osmosis cartilages and the treatment of emission-gas after industrial coal burning. An example for the removing of (only) bio-degradable dissolved substances is the treatment of water discharged for dairy products production facilities.
The removal of SS-particles from fluids and gases, referred to as “filtering”, is done by passing the fluids and/or gases through a porous martial. Of the many porous media used, fabrics are especially common.
The separation ability (the filtering capability or degree) of fabrics depends on their thread density which, in turn, defines the density of pores in a give area. The number of threads per linear inch, defined by the term “mesh”, is often used to describe the filtering degree of fabric filters. Another term used to describe the nominal sieving or filtering degree is an actual linear dimension of the shortest straight-line distance (length or width) across an individual opening or pore of the filter medium. This is most often given in microns. The absolute filtration degree is the length of the longest straight-line distance across an individual opening of the filter medium.
When comparing filters the term “open area” is used. The open-area is the pore area or sum of all the areas of all the holes in the filter medium through which the fluid can pass. Filtration open area is expressed as a percentage of the effective filtration area.
In using a porous filter (fabric or other proliferated medium) the “open area” gradually decreases with the accumulation of suspended particles a layer of particles is formed, (referred to as “filtering cake”) till the filter is completely blocked. A back-flow (referred to as “backwash”) of a liquid or a gas through the filter in the opposite direction of the accumulation of the particles will remove the cake and refresh the filter. Backwashing is effective if the filtered-out particles have not been strongly attached to the filtering medium.
Porous filtering medium, when clean, have enough open area to cause insignificant pressure drops across the medium. However, as suspended SS-particles begin to plug up openings the available open area for the fixed flow rate to pass through decreases, leading to a gradual increase in the stream-through velocity through the medium. Since the pressure drop is proportional to the square of this velocity, the differential pressure across the medium will increase over time as an exponential function. Less open area also means less SS-particles required to increase pressure drop across the medium. The type of weave or knit used to construct a fabric filter can affect the open area greatly. Less open area also means less SS-particles required to increase pressure drop across the fabric element. The type of weave or knit used to construct a fabric filter can affect the open area greatly.
Focus is now turned to the aspect of removal of bio-degradable substances utilizing a porous medium:
When bio-degradable substances dissolved in a microorganisms-supporting-liquid, typically in a water solution, come into contact with a solid surface medium, microorganisms develop over the surfaces of the medium. In a gaseous material, bio-degradable substances can be dissolved in the gaseous vapor or droplets of a microorganisms-supporting-liquid that constitute part the gaseous material. As is the case for liquids, when the bio-degradable dissolved substances in a gas material, typically in water vapor or droplets, come into contact with a solid surface medium, microorganisms develop over the surfaces of the medium. The rate and type of growth depends on the length exposure time as well as on the characteristics and concentration of the dissolved substances, the dissolving material and on many environmental-growth parameters such as the composition of the medium, the temperature, the moisture and the pH. As the microorganisms develop they utilize for their multiplication and biomass-maintenance the dissolved substances—thus removing the substances from the dissolving fluid-material. Biofilm is typically formed by the utilization of dissolved organic substances. The larger the surface area available for the development of microorganisms per volume of a porous medium the more efficient is the removal of the bio-degradable dissolved substances. The growth of the microorganisms is manifested in a mucilaginous protective coating layer in which dead and living bacteria and fungi are encased. As the coating, referred from herein after as “a biofilm”, develops and thickens it gradually clogs passages and pores when it develops in a porous medium.
While SS-particles particles typically clog porous filters by forming a cake on the external surface of the receiving-side of a filtering medium, biofilm develops over all the exposed surfaces of the porous filtering medium.
The ability to remove dissolved substances from liquids and vapor by densely growing microorganisms in biofilm is favorably utilized in a wide range of devices. The devices are based on a porous medium having large and dense surface-areas exposed to the passing streams of liquid or gas containing the dissolved substances. With the increase in compaction of pours and passages, the medium becomes more readily clogged by biofilm.
In many cases the passing of either a liquid or a gas material through a porous medium causes both the accumulation of clogging SS-particles and the development of biofilm.
As the SS-particles and biofilm accumulate in the course of time (either simultaneously of separately) the narrow passages through the porous medium clog. Refreshing of the medium is typically done by flushing the medium in the opposite direction of the initial operating direction. The flushing is done with a strong liquid or gaseous stream. The tighter the SS-particles are embedded and biofilm enmeshed on and in a porous medium, more energy and efforts are required for the porous medium refreshing.
In prior art different devices and methods to make backwashing efficient have been disclosed. Examples of such devices are given in U.S. Pat. No. 6,136,202 (Foreman) and WO2005/021140 (Johnson et al. The patents describe techniques of removing the SS particles by applying water jet (Foreman) and air-bubbling (Johnson et al.) forces.
Examples of media sheets with passages between them for growing biofilm for water purification is given in U.S. Pat. No. 5,388,316, U.S. Pat. No. 5,430,925 (MacLaren) and US Patent Application 2003/0104192 (Hester et al.). The use of threads and fibers for growing biofilm is disclosed in U.S. Pat. No. 5,389,247 (Woods), U.S. Pat. No. 5,262,051 (Iwatsuka) and U.S. Pat. No. 6,190,555 (Kondo). Once biofilm has developed and the organic substances removal becomes ineffective the medium has to be refreshed by energetic backwashing or/and physical scraping (accompanied at times by chemical treatments).
Another aspect of purifying liquids, typically wastewater, is the use of loose floating particles with large surface area for biofilm development. In the explanation that follows the use of floating particles is given in reference to wastewater but the use of particles can be made in other microbiological supporting liquids. The floating particles, referred from herein after as “free-drifting particles”, are small particles with a density slightly lower than water that are kept suspended in the water by air diffusers or mechanical mixers are described in U.S. Pat. No. 5,458,779 (Odegaard) and are known as the Kaldnes Moving Bed Reactor (KMB) or the NATRIX Technology. A refinement in the use of the KMB technology is described by Shechter et al. in U.S. Pat. No. 6,616,845 in which suspended inert free-drifting particles are used in conjunction with vertical partition elements to control the free movement of the particles. The particles used in both patents are made plastic material having irregular shape with large porosity.
Water purification effectiveness of carrier particles diminishes as biofilm develops and clogs the water passages within the particles. To remedy the clogging the suspended particles have to be periodically treated. Treatment is typically done by gathering the particles and mechanically or chemically removing the biofilm prior to re-use or replacing clogged particles with new ones. Both options are time consuming and expensive.
Typically the structures of both SS-particles removing media (fabric filters and plate-surface filters made of inert materials) and the structure of media for intentionally growing biofilm (such as stacked sheets made of inert materials with spaces between them such as packed threads and fibers) are kept in a fixed state throughout the cycle of accumulation and backwash procedure for the refreshing of the media. The “open-area” and distance between the threads and fibers in the medium maintain the initial ratio throughout the operational life of the media.
Amongst commonly used filtering media, knit fabrics are widely used. An example of such use is given in DE102005023150 (Sabine) which describes a filter sock for removing dirt particles from a fluid comprises a wire-reinforced tube of knitted fabric. An independent claim in the patent includes a filter sock with a filter fabric layer formed by circular knitting and incorporating a reinforcing wire into the filter layer. Another example is DE102004020848 (Hans-Joaachim and Diether) which discloses a filter sock for removing dirt particles from a liquid, having a tubular filter layer of knitted fabric and includes wire reinforcement attached to the filter layer. An independent claim in the patent includes a filter device includes a filter sock located in a hollow profile (specifically a tube) with several radial openings. In both quoted patents the configuration of the knit fabric (the structural configuration between the filaments of the fabric) does not change in the course of using and cleaning of the filtering medium.
It is the aim of the present invention to disclose a method and a device for the removing of contaminants from fluid-materials by a substrate that can be easily and efficiently refreshed by stretching and backwashing when it becomes clogged.